Films extruded from high density polyolefins, especially high density polyethylene, have become an important product worldwide. Even more particularly, the use of multimodal or bimodal polyethylene in films has gained market share over traditional unimodal polyethylene. In order to make such films, complex reaction processes such as the use of multi-site catalysts and/or multi-stage reactors are necessary. This can often lead to problems in the quality of the film, such as the presence of gels. Gels are highly localized imperfections in the film that are visually distinct from the surrounding film. There are typically several different causes of gels, including unblended polymer, unreacted catalyst and activator, localized areas of varying molecular weight, localized areas of crosslinked material, and contamination (e.g., inorganics, sand, carbon black, etc.) that can reflect and transmit light differently from the surrounding polymer. The presence of gels can lower the value of these films and, in some cases, makes the films unmarketable.
A number of different techniques may be used to reduce gel formation in polyolefins. Careful control of upstream systems and extrusion temperatures can prevent contaminants such as fibers and degraded polymer or black specks that cause gels. While these techniques may be suitable for reducing certain types of gels, improved techniques for reduction of gels in polyolefins are needed, especially those techniques that can reduce gels due to localized areas of varying molecular weight which can be hard to remove.